FIG. 1 of the accompanying drawings schematically illustrates a conventional butterfly valve 10. A rotatable shaft 12 extending through a plate-type flow modulator 14 is supported by a mounting body 16. The mounting body 16 forms part of a duct 18 which defines a flow path 20. Flow proceeds in the direction indicated by the arrow 22. When the modulator 14 is in an open position as illustrated, high-pressure fluid exerts a closing torque on the high-pressure side 24 of the modulator. If static pressure is measured at several locations from the leading edge 26 to the trailing edge 28 of the modulator 14, it is observed that the pressure is substantially uniform over a range represented by a series of equal-length arrows 30. A drop-off in static pressure is observed over a smaller range near the trailing edge 28, this being represented by a series of arrows 32 having successively smaller lengths. This drop-off in pressure apparently accounts for the closing torque exerted on the modulator 14 by the high-pressure fluid. In order to maintain the position of the modulator 14, an opening torque must be continuously applied to the shaft 12. Typically, the torque is applied to the shaft 12 by an external actuator (not shown). If torque exerted by the fluid can be sufficiently decreased by changing the design of the modulator 14, then a smaller actuator may be used to provide opening torque. This is particularly important in applications such as cabin pressurization systems for aircraft, where weight and volume reduction is an on-going goal.
A known expedient for significantly reducing the closing torque exerted by the fluid is to provide a protruding portion 34 (hereinafter "trailing fence") of the modulator 14 on the high-pressure side 24 near the trailing edge 28, as illustrated in FIG. 1A. The trailing fence 34 effectively reduces the range over which the pressure drop-off represented by the arrows 32 (FIG. 1) is observed.
In addition, it is known that closing torque can be altered by providing a protruding portion 36 (hereinafter "leading fence") on the low-pressure side 38 of the modulator 14 near the leading edge 26. However, the use of such a leading fence 36 alone (i.e. without a trailing fence 34) has been avoided because closing torque is thereby increased. When used in combination with a trailing fence 34 as illustrated in FIG. 1A, the leading fence 36 generally increases closing torque over that obtained through use of the trailing fence 34 only, and only slightly decreases closing torque under that obtained with a conventional modulator having no fences. However, this slight reduction is achieved at the expense of a considerable reduction in flow rate. Accordingly, the use of a leading fence 36 has not been well received either alone or in combination with a trailing fence 34.
An objective of the invention is to provide a plural-fence, plate-type flow modulator for a butterfly valve whereby in operation of the valve a substantial reduction in the aerodynamic or hydrodynamic torque exerted on the modulator is realized.